Triglyceride-Glucose Index and Cardiovascular Events in Kidney Transplant Recipients

Introduction Kidney transplant recipients (KTRs) have an increased risk of cardiovascular (CV) events (CVEs) compared with the general population. The impact of insulin resistance on CV risk after transplantation is not well defined. Methods We tested whether triglyceride-glucose (TyG) index, a surrogate marker of insulin resistance, may predict posttransplant CVEs in a cohort of 715 consecutive KTRs all included 1 year after transplant. Results Follow-up was 9.1 ± 4.6 years. Mean TyG at inclusion was 4.75 ± 0.29 (median, 4.73 [4.14–5.84]). In multiple regression analysis, having a TyG above the median value was associated with higher body mass index (BMI), low high-density lipoprotein (HDL) cholesterol level, and greater urinary protein excretion. A total of 127 CVEs (17.7%) occurred during the study period. In univariate analysis, TyG was strongly associated with CVE occurrence (hazard ratio [HR] 2.06, 95% CI 1.42–3.50, for each increase of 0.1 in TyG, P < 0.001). The best predictive value was 4.87 (HR 6.32, 95% CI 3.30–12.11, P < 0.001). The risk of CVE gradually increased with higher TyG index (quartile 2, HR 1.71, 95% CI 0.84–5.20, P = 0.139; quartile 3, HR 3.12, 95% CI 1.61–6.02, P < 0.001; quartile 4, HR 7.46, 95% CI 4.03–13.80, P < 0.001, vs. quartile 1). TyG remained associated with CVE in multivariate analysis (HR 2.11, 95% CI 1.22–3.68, for each increase of 0.1 in TyG, P < 0.001). Conclusion Insulin resistance, as measured by the TyG index is strongly associated with CVE in KTRs. Improving insulin sensitivity seems to be a major issue to prevent CV morbidity and mortality in this high-risk population.

K TRs have an increased risk of CVEs compared with the general population. 1 This is, in great part, linked to a very high frequency of traditional CV risk factors. 2,3A great number of traditional CV risk factors, such as diabetes, low HDL cholesterol, or hypertension, are related to insulin resistance.Although insulin resistance is likely to be frequent after kidney transplantation, reliable measure of frequency is lacking.Reduced renal function and immunosuppressive drugs are some factors which could participate in the high prevalence of insulin resistance in KTRs. 4 In addition, metabolic syndrome (MS), which is closely linked to insulin resistance, is observed in one-third of KTRs 1 year posttransplant and is associated with CVE. 5 Moreover, a large part of KTRs had either diabetes before transplantation or develop de novo diabetes after transplantation, 6 and diabetes and even prediabetes are associated with the occurrence of posttransplant CVE. 7evertheless, although all these examples suggest the role of insulin resistance, they relate to more than only insulin resistance.Indeed, MS also depends on chronic low-grade inflammation which indisputably contributes to CVD in KTRs. 8However, posttransplant diabetes is mainly related to beta cell dysfunction rather than insulin resistance. 9,10yG index is a simple, reliable, and objective surrogate marker of insulin resistance, and some studies suggest that it is a better marker of insulin resistance than Homeostatic Model Assessment for Insulin Resistance. 11,12In this study, we tested whether TyG index may predict posttransplant CVE in a large cohort of KTRs with a follow-up of almost 10 years.

Study Design and Populations
Research has been conducted in 715 consecutive patients transplanted between January 1, 2004, and December 31, 2018, in the Transplant Unit of the CHU of Besancon and having at least 1 year follow-up after the transplant.A total of 65 patients who died or lost their graft in the first-year posttransplant had been excluded.
Among these 715 KTRs, 221 patients (31%) had received T cell-depleting antithymocyte globulin therapy and 494 (69%) had received nondepleting a-CD25 monoclonal antibody therapy.Tacrolimus and mycophenolate mofetil were used as first immunosuppressive regimen in all patients.Targeted trough levels of tacrolimus were between 6 and 8 ng/ml during the first year and between 4 and 6 ng/ml after (except in cases of acute rejection in the first-year posttransplant or for those having developed donor-specific antibodies).Steroids were withdrawn between the thirdand the sixth-month posttransplant (except for those who underwent acute rejection before 3 months).All the transplants were performed with a negative crossmatch.
Cytomegalovirus prophylaxis was given according to a standardized protocol.All cytomegalovirusexposed patients received valganciclovir for 3 months.All cytomegalovirus-naive patients having received a cytomegalovirus-positive kidney received valganciclovir for 3 or 6 months.All patients received Pneumocystis antimicrobial prophylaxis with trimethoprim-sulfamethoxazole for at least 6 months.
Database was closed for analysis on December 31, 2021.Minimum follow-up was 3-year posttransplant.

TyG Index
The TyG index was calculated by using the following formula: Ln (TG [mg/dl] Â fasting glucose [mg/dl]/2). 13nfounding Factors Age, sex, BMI, abdominal circumference, diabetes, hypertension (treatment with antihypertensive drugs), statin use, smoking habit, immunosuppressive drugs were analyzed as covariates.Concerning abdominal obesity, waist circumference was measured during dietetic consultation.Abdominal obesity was defined as a waist circumference of more than 88 cm in women and more than 102 in men.MS was present when at least 3 among the following 5 conditions were observed: waist circumference >88 cm in women or >102 cm in men; triglyceride level above 150 mg/dl; hypertension; HDL cholesterol under 50 mg/dl in women or 40 mg/dl in men; and fasting glucose level above 100 mg/dl.Dialysis mode (none, hemodialysis, or peritoneal dialysis) and its duration before transplantation were also recorded.Delayed graft function and acute rejection occurred during the first-year posttransplant.Other relevant immunologic parameters, such as pretransplant panel-reactive antibodies (0 vs. positive panelreactive antibodies at any level), current donorspecific antibodies, and transplant rank (first vs. second or more), were analyzed as covariates.Diverse biochemical parameters were also analyzed (total cholesterol, HDL cholesterol, creatinine).Urine protein/ creatinine ratio was assessed as the protein/creatinine ratio (g/g).Glomerular filtration rate was calculated with the CKD-Epidemiology Collaboration equation. 14DL cholesterol was estimated through the Friedewald formula. 15ethods of assessment and definitions of these variables have been previously described in detail. 16tcomes The primary end point was the effect of TyG on CV event-free survival defined as the time elapsed between 1-year post-transplant and the first CVE.Atherosclerotic events were considered as previously reported. 8yocardial infarction was documented by serial 12lead electrocardiogram evidence or Q-wave infarction and appropriate myocardial enzyme level elevations, coronary revascularization including coronary artery bypass surgery or percutaneous transluminal coronary angioplasty, or typical history of angina with abnormal coronarography result.
Both nonhemorrhagic and hemorrhagic strokes were confirmed by neurologic examination findings consistent with new-onset focal neurologic deficits, with or without computed tomography or magnetic resonance imaging evidence of cerebral infarction or symptomatic extracranial artery stenosis resulting in carotid endarterectomy.
Doppler or arteriography findings were used to confirm the presence of abdominal aortic repair, lower extremity revascularization through bypass surgery or angioplasty, lower extremity amputation, or new onset of intermittent claudication.Two physicians independent of the study were responsible for diagnostic ascertainment.This analysis was performed without knowledge of baseline characteristics.CVEs were retrospectively recorded.

Statistical Analysis
Median (interquartile range), mean values (SD), and frequency (percentage) were provided for continuous and categorical variables, respectively.Medians, means, and proportions were compared using Student t-test and chi-square test (or Fisher exact test, if appropriate), respectively.Correlations between variables were assessed by Pearson correlation coefficient.
Receiving operator characteristic curve was used to determine different diagnostic abilities.
Follow-up duration was calculated using a reverse Kaplan-Meier estimation.
Cox proportional hazard models were performed to estimate HR and 95% CI for factors associated with CVE.The association of baseline parameters with the occurrence of a CVE was first assessed using univariate Cox analyses, and then parameters with P values < 0.05 were entered into a final multivariable Cox regression model.The assumption of proportionality was checked by plotting log-minus-log survival curves and by cumulative martingale process plots.

Characteristics of the Study Population
Characteristics of the study population are depicted in Tables 1 and 2. Mean follow-up was 9.1 AE 4.6 years.
There were 329 patients (46%) who met the criteria for MS.TyG index was higher in patients with MS (4.94 AE 0.25 vs. 4.59 AE 0.20, P < 0.001).Area under the curve of receiving operator characteristic curve was 0.88 (0.86-0.91) (P < 0.001).A TyG index of 4.74 was the best predictive value with a sensitivity of 84% and a specificity of 82%.
TyG was not associated with any immunosuppressive drug.TyG was similar in patients receiving statins and in those not receiving statins (4.73 AE 0.30 vs. 4.75 AE 0.28, P ¼ 0.611).
Even after adjustment for MS, TyG index remained associated with CVE (HR 1.84, 95% CI 1.25-3.45,for each increase of 0.1 in TyG, P < 0.001).The incidence of CVE in patients with MS but with TyG index under the median value was 7.8% whereas it was 15.6% and 29.5% in patients with TyG index value above the median without and with MS, respectively.
Subanalysis in different subcategories of patients confirmed the association between TyG and CVE even when this association seems to be lesser in patients with diabetes (Table 3).

DISCUSSION
This study cohort reports that a higher TyG index is associated with higher incidence of post-transplant CVEs.The association is independent of all other major CV risk factors.Subgroup analyses confirmed the association between TyG index and CVE in all patients' categories.We observed these results in a large cohort of patients with a long follow-up.Finally, diagnostic ascertainment was independent of knowledge of TyG values.This finding reveals that high TyG, a surrogate marker of insulin resistance, is a potent risk factor for CV morbidity in KTR.
Our results are consistent with those of most previous studies in other patients' population.
][19] However, a recent study in Italy described similar association between TyG and incident cardiac events in patients with known or suspected coronary heart disease. 20Recently, Lopez-Jaramillo et al. 21reported on the association of TyG index with mortality and CV diseases in individuals from 5 continents.They observed that higher levels of TyG were associated with CV end points only in patients from low-or middle-income countries.This may suggest that the impact of insulin resistance on CV outcomes could be different according to income or genetic background.
However, TyG was associated with major cardiac events in all studies including patients with chronic kidney disease. 22,23Moreover, previous studies, using  different surrogate markers of insulin resistance, have been associated with the occurrence of CVE in patients with CKD. 24Altogether, these results and ours suggest that physicians should have a special attention to insulin resistance as a CV risk factor.A major issue for the clinical use of a marker is to determine a threshold.A threshold of 4.87 had the best predictive value for CVE occurrence and included 35% of the study population.
In the general population, a TyG value above 4.49 was found to be the best cutoff to define insulin resistance. 25In this cohort, almost 80% of patients had a TyG index above 4.49 illustrating the frequency of insulin resistance.This may partly explain the high incidence of CVEs in KTRs.Insulin resistance contributes to CV damages in many ways, some direct and others indirect.Insulin is crucial to maintain vascular integrity by supporting nitric oxide synthesis in endothelial cells and inhibiting growth and migration of vascular smooth muscle cell. 26Moreover, hyperinsulinemia increases the production and release of endothelin-1 by endothelial cells. 26Consequently, insulin resistance is associated with an imbalance between nitric oxide and endothelin-1 resulting in vasoconstriction and proliferation of vascular smooth muscle cells.Insulin resistance is also associated with low-grade inflammation.Indeed, white adipose tissue produces several adipocytokines with inflammatory properties such as interleukin-6, tumor necrosis factora, and monocyte chemoattractant protein-1 among others. 27,28Thus, we previously revealed that interleukin-6 levels were higher and that interleukin-6 production capacity was associated with incident CVEs in obese KTRs. 29In parallel, insulin resistance is associated with low HDL cholesterol and hypertension that may contribute to increase CV risk. 30Consequently, insulin resistance is per se responsible for vascular dysfunction and is associated with pro-atherogenic conditions.
Although the pathogenesis of MS has not been clearly elucidated, insulin resistance and chronic lowgrade inflammation derived from central obesity are the most widely accepted as underlying pathophysiology.We previously reported that MS was a strong risk factor for incident CVEs in KTRs. 5 TyG is also a strong surrogate marker of MS in this population.However, TyG index is better associated with CVE than MS itself.Indeed, increasing levels of TyG are also predictive of CVE in patients without MS.Patients with MS exhibit low-grade inflammation as measured by CRP.We observed a correlation between TyG index and CRP.However, TyG predicted CVE independently of inflammation.
Except smoking, TyG index was the only modifiable factor associated with CVE in this transplant cohort with most patients already under statins.As TyG index is mainly a surrogate marker of insulin resistance, interventions aiming to enhance insulin sensitivity should be encouraged and tested in future trials.In this strategy, weight reduction and increase in exercise are  31,32 However, in these studies, no clear effects were observed on glucose metabolism.Nevertheless, in patients with early type 2 diabetes, intensive lifestyle intervention allows significant weight loss and is associated with diabetes remission in approximately 50% of patients. 33,34However, these measures seem to have to be very intense to be effective.Of note, both vegetable intake and Mediterranean diet style are associated with a decreased risk of post-transplantation diabetes, suggesting a favorable effect on insulin sensitivity. 35,36Some drugs act as improving insulin sensitivity.However, their use is restrained to patients with diabetes.Hypertriglyceridemia could be a therapeutic strategy.The REDUCE-IT trial 37 revealed that the addition of icosapent ethyl, a highly purified form of eicosapentaenoic acid, reduced ischemic events among statin-treated individuals with elevated triglyceride levels and high CV risk.Nevertheless, in this study, greater TG lowering was not predictive of better CV prevention.Studies testing niacin or omega-3 to reduce CVE failed although significant decrease in TG was observed. 38,39Finally, a large meta-analysis revealed that fibrates could be moderately effective in secondary prevention. 40Altogether, these results suggest that although TG and TyG are potential biomarker of CV risk, there are few evidences that TG lowering by itself is an effective strategy for reducing such risk.
A large part of the patients was under statins (75%).Those patients were more prone to be older and to have diabetes and/or a previous history of CVE.An indication bias is consequently obvious making the association between LDL cholesterol and CVE difficult to assess.Nevertheless, we observed a strong positive association between LDL cholesterol and incident CVE in patients not receiving statins (data not revealed).Furthermore, we only included patients having a functional graft 1-year posttransplant.Even when it is by the way a survival cohort, we previously reported that metabolic parameters drastically change during the first-year posttransplant making interpretation of pretransplant parameters impossible.
Insulin resistance, as measured by the TyG index, is strongly associated with CVEs in KTRs.Improving insulin sensitivity seems to be a major issue to prevent CV morbidity and mortality in this high-risk population.

DISCLOSURE
All authors declare no support from any organizations for the submitted work; no financial relationships with any organizations that might have in interest in the submitted work in the previous 3 years; and no other relationships or activities that could seem to have influenced the submitted work.

Table 2 .
Biological characteristics at the entry in the study

Table 3 .
Association between TyG and cardiovascular events in different patients' categories eGFR, estimated glomerular filtration rate; HR, hazard ratio; TyG, triglyceride-glucose.

Table 4 .
Factors associated with cardiovascular events Regular exercise and dietary interventions are associated with weight loss and reduction of fat mass in KTRs.
a For 1-year increase in age.b Male vs. female.c For each increase of 1 kg/m 2 .d For each increase of 10 mg/dl.e For each increase of 10 mmol/l.f For each increase of 1 ml/min per 1.73 m 2 .g For each increase on 0.1 g/g of creatinine.M Colladant et al.: TyG Index in Kidney Transplantation CLINICAL RESEARCH Kidney International Reports (2023) 8, 2307-2314 crucial.